In an extrusion process, polymer pellets are heated and melted as they are driven through a barrel by an extrusion screw. Plastic extruders generally require the filtration of impurities from the molten polymer exiting the extruder screw with a filter such as a screen in order to produce a high quality product. Once the filter has become dirty by trapping a certain amount of impurities, it is more difficult for the polymer to pass through the filter. As a result, the pressure of the polymer upstream from the filter increases and output decreases.
Various screen changing devices are available to replace dirty filters with clean filters. A well-known type of screen changer consists of a hydraulically operated slide in which two filter assemblies are located. One filter assembly is positioned on-line within the bore of the extruder to filter polymer while the other filter assembly is off-line external to the bore. In order to replace the dirty on-line filter with the clean off-line filter, the slide is shifted, thereby moving the clean filter on-line with the bore and moving the dirty filter off-line for replacement. A problem with conventional hydraulic screen changers is that there is a momentary leakage of polymer resulting in loss of pressure and flow while the slide is being shifted.
A solution to the flow and pressure interruption was proposed to allow for continuous flow operation with filter changing. That solution provided an array of filter modules adapted to slide through the extruder barrel, with only one filter positioned in the bore at a time. To allow for proper sealing during filter changes, a wide sealing region was provided to either side of the bore. A clean filter would shift into one sealing region as a clean filter shifted into the bore, a dirty filter shifted into the other sealing region, and yet another dirty filter shifted out of the barrel. The latter filter module would be removed, the filter could be changed and the module would be replaced at the clean filter side of the barrel. The need for manual handling of the filter modules proved cumbersome and undesirable.
A second solution uses a screen changer having dual bores and two sliding filters. During a normal operation, polymer flows through both bores and passes through both filters. When the filters become dirty, the filters are changed one bore at a time. A slide moves the dirty filter through a seal region to a location outside of the extruder barrel for changing. The polymer can flow through the one bore and remain continuous while the other bore is closed off by the slide.